AthenaKernel: Fix some references to SGTools.
Fix a few references we had to headers in SGTools which have been moved to AthenaKernel.
Also remove references to CxxUtils/unused.h and CxxUtils/make_unique.h, both of which are now obsolete.
Fix a few references we had to headers in SGTools which have been moved to AthenaKernel.
Also remove references to CxxUtils/unused.h and CxxUtils/make_unique.h, both of which are now obsolete.